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Unlike the sit-ups, "The Crunch", or curl-up, considered to be very effective to form the stomach muscles, especially the muscle called the rectus abdominis (the front of the muscle belly). This is the muscle that appears on the belly of the six-pack. Maybe you actually have to do the crunch, only you and your friends still call the sit-up. On this second movement of the hands are placed behind the head, but not too tight because the neck can cause injury and interested. Variations on the other hand can in cross on the front of the body with both hands touching the shoulders.

The difference with sit-ups, the crunch on the back bottom center of the backbone to the tail bone does not follow up. In other words, this exercise will be harder to train your stomach muscles by reducing the risk of injury to your back.

To be effective, the fitness instructor you usually will suggest to you to the navel, or seems to suck up your stomach, during a crunch. In fact it is less effective, even a research has proven that with the "suck your stomach" rectus abdominis activity will be reduced.

Then do the exercise as an effective stomach muscle?
One study involving 25 people shows that crunch will be more effective if the stomach is tight. Thus the navel should be driven out with a muscle strain, muscle belly.

To reduce the risk of back and neck injuries during a crunch, one of the legs can be folded up to form a 90o angle while the other leg remains straight in parallel with the floor. In addition, place your tongue on the roof top of the mouth to help stabilize the neck muscles.